1. Field of the Invention
The present application concerns medical devices for treatment of vascular disease and more particularly devices for treatment of abdominal aortic aneurysms.
2. Description of Related Art
Most of us are familiar with the problems of vascular blockage brought on by high fat diets, smoking and other risky behaviors. Generally fatty or other lesions block the vasculature requiring surgical replacement or unclogging (e.g., angioplasty) to restore blood flow. Such problems are common in the vasculature of the heart where blockage can result in "heart attacks". However, vascular narrowing and blockage is also common in the extremities (e.g., restriction of blood flow into a leg) as well as the vasculature supplying blood to the brain where blockage can result in a stroke.
While one does not normally think of these types of blockages occurring in the main artery (aorta) carrying blood away from the heart, other, possibly related, serious types of vessel disease do take place in the aortas. The abdominal aorta is the major artery carrying blood posteriorly from the heart and normally has a diameter two to two and one half centimeters in an adult. The aorta extends in a relatively straight path from the heart toward the groin and then bifurcates to supply blood to the legs. Perhaps because of its size and the volume of blood that moves through this vessel, fatty blockages and thromboses are not as common in this vessel. Rather, vascular disease often resulting from genetics, smoking and high blood pressure cause a weakening of the aorta's walls and a resulting distension.
Such distensions are known as an abdominal aortic aneurysms (AAA) when they occur in the aorta from the renal arteries down to the bifurcation to form the iliac arteries. At first an aneurysm is quite small but as the disease process continues, the aneurysm enlarges, the aorta wall thins and rupture ultimately results. When the aneurysm is less than 4.5 cm in diameter, danger of rupture is quite low. Even before the aneurysm grows large enough to pose a danger of rupture, however, it may cause other problems. The enlarged region often develops a thrombus that fills the distension so that blood flows only down the central region. Pieces of clot may break off from the thrombus and be carried away, resulting in blockages in the legs, lungs or even the brain.
The aneurysm generally does not remain small but enlarges at a rate of 0.3-0.5 cm per year. An 8 cm aneurysm has a 75% per year rupture risk. Needless to say rupture of such a major vessel is often fatal. About 15,000 people die each year in the United States from ruptured AAA's. If rupture occurs, 62% of the victims die before reaching a hospital. Of those surviving long enough to undergo surgery another 50% die. Even if the aneurysm is discovered before rupture, surgical repair is difficult and risky although surgery is 95% successful.
Traditional repair methods require full abdominal surgery with protracted recovery periods. Further, many weakened patients with heart disease or other maladies cannot be subjected to the rigor of such surgery. Therefore, many people are trying to develop an "endovascular" repair technique in which a prostheses is introduced into the aneurysm not by opening the patient's abdomen but by remote insertion of a femoral artery. After insertion the device is advanced to the aneurysm where it is deployed to repair the AAA. Clearly such a technique would significantly reduce patient complications and recovery times.
Much stenotic vascular disease is treated with stents--usually metallic meshes intended to force open a vessel. Simple stents are not ideal for AAA because the thrombus readily penetrates the open mesh of the stent and because blood passes through the mesh to place continued pressure on the aorta wall.
The other device common in vascular repair is a synthetic vascular graft made of expanded polytetrafluoroethylene (ePTFE). An advantage of these synthetic grafts is that they are extremely flexible and can be readily compressed to a very small size for endovascular insertion. However, bypassing generally requires suturing of the graft to the patient's vessels. This suturing is not possible with an endovascular insertion. Thus, if a synthetic graft is compressed and then inserted endovascularly into a AAA, it is unlikely that the graft will unfurl, anchor to the aorta and remain properly in place to repair the aneurysm.
Most current AAA devices combine a synthetic graft component with some type of a stent device. The graft is intended to exclude the thrombus and reinforce the aortal wall while the stent device ensures proper opening and anchoring of the device. Typical of such a device is that disclosed in U.S. Pat. No. 5,275,622 to Lazarus, which is a tubular collapsible graft, having a mechanical framework at its ends. Not only does the framework ensure proper opening of the tubular graft, it can also have barb-like anchors that fasten the graft to the walls of the vessel. This reference illustrates an unbranched prosthesis but U.S. Pat. No. 5,489,295 to Pilani et al. shows a bifurcated prosthesis comprising a tubular graft and a supporting stent structure along with a delivery system. U.S. Pat. No. 5,360,443 to Barone et al. discloses another version of an aneurysm repair prosthesis comprising a stent covered by a synthetic graft.
Successful endovascular AAA prostheses must meet a number of criteria. First, the graft wall material of the device must have sufficient strength to withstand the force of the blood flowing through the aorta. Second the device must become firmly and permanently anchored in the aorta. If the anchoring is inadequate, blood will leak around the graft and the device will ultimately fail. Third, the device must be sufficiently compressible to allow endovascular insertion. To some extent these factors work at cross purposes. If the graft material is thickened to ensure adequate strength, the device will have a larger compressed profile. If additional stents are added to improve the anchoring of the device and to enhance the strength of the device, the compressed profile will again be increased.
A number of prior art attempts have been made to make the graft material also act as a stent. U.S. Pat. No. 5,156,620 to Pigott discloses a semi-rigid tubular prosthesis with double walls. Following insertion a hardening polymer can be injected between the walls to permanently stiffen the device. U.S. Pat. No. 5,607,468 to Rogers et al. discloses an inflatable corrugated stent graft with an internal structure not unlike an air mattress. Such a device can be compressed for delivery and then expanded by injection of a liquid or gas. However, it appears that neither of these devices can be compressed to an extremely low profile. The bifurcated graft disclosed in U.S. Pat. No. 5,693,088 to Lazarus employs inflatable collars, primarily at the extremities of a bifurcated graft to ensure that the device is sealingly anchored in place. U.S. Pat. No. 5,665,117 to Rhodes combines a number of these features. A tubular graft is equipped with a stiffening stent and also surrounded by an inflatable balloon. Once the device is delivered to the aneurysm and the stent enlarged to hold open the tubular graft, the balloon can be inflated to occupy the peripheries of the aneurysm thereby anchoring the device in place.